Thermistors vs Thermocouples

Thermistors vs. Thermocouples – What’s the Difference?

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Thermocouples and thermistors are two words that everyone has probably heard at least once in their Physics class.

While it was tough to understand the differences back in high school, it is quite simple, provided you are familiar with both.

Even if you are not, this article on thermistors vs. thermocouples will give you the gist of things without too much jargon.

Both thermistors and thermocouples are used widely in homes and industries, so it’s worth knowing the differences.

Furthermore, they are pretty useful where needed, and you will see that they are needed in lots of places.

So, fear no more, because we will explain the two in the easiest way possible!

General Information

Before we get to the differences between a thermistor and a thermocouple, let us first look at the two separately.

Here we will discuss the two items in perspective and briefly explain how they work before moving to the crux of the issue, i.e., the differences.

1. Thermistor

We start with thermistors and what they actually are. A thermistor is the shortened version of the full title. Thermistors are officially called thermal resistors.

Why are they called thermal resistors? Well, this is to do with the resistance of the item. The resistance of the thermistor is its ability to resist the flow of electricity through it.

For a thermistor, this resistance is sensitive to changes in temperature. Hence, we can say that a thermistor is a circuit unit whose resistance is dependent on the temperature change.

This particular property comes in handy because it allows fluctuations in temperature to be measured with a great deal of precision.

Moreover, thermistors are set in the electrical circuit, so they are easy to install or fix.

Basic Construction

The basic construction principle of thermistors is the same almost everywhere. They consist of a special semiconducting powder that is mixed to harden.

This hardened mixture is then formed over lead wires. Once the mixture hardens over the lead wires, the construction is inserted into a furnace for drying.

Once the entire mixture has been dried off and has been hardened sufficiently enough, a glass cover is put over the thermistor. This glass cover acts to protect the delicate mixture inside.

The maximum size of a thermistor is around 1.5mm, and they are available in different values. The resistance of these thermistors ranges from 1K to 100K. This K indicates the resistance of the thermistor at room temperature.

While they are available in various sizes, such as rod type, disc type, and so on, the rod type is one of the most common types used.

Types of Thermistors

When it comes to types, there are fundamentally two types of thermistors. They are the Negative Temperature Coefficient (NTC) Thermistor and the Positive Temperature Coefficient (PTC) Thermistor.

  • NTC Thermistor

The NTC thermistor is the type of thermistor whose resistance falls when the surrounding temperature increases. You can understand that the relationship between temperature and resistance is inversely related.

This type of thermistor is perhaps the most common type found in different circuits and is quite popular, too, due to its stability and robustness.

  • PTC Thermistor

The second type of thermistor is the PTC thermistor, and when it comes to its working principle, it is opposite to the NTC thermistor. It means the resistance of this device increases with the subsequent rise in temperature.

Hence, the opposite is also true. That is, when the temperature falls, the resistance of the thermistor also falls. PTC thermistors are not as common as their NTC counterparts. The primary use of PTC thermistors is their application in circuit protection.

When the current flow through a PTC thermistor increases, it will cause the thermistor to heat up. When this happens, the resistance also rises, and this subsequently limits the current flow through the circuit as well.

From here, you can see that PTC transistors are mainly used to protect the circuit.

Where Are They Used?

When it comes to its uses, thermistors are very useful, and they are now an integral part of the electronics industry.

Some of their most common uses include:

  • Digital thermometers
  • Measuring oil temperatures in vehicles
  • They are an integral part of ovens
  • Protecting circuits from current fluctuations
  • Found in batteries
  • Arduino kits and circuits.

2. Thermocouple

Now that we have discussed quite a lot about thermistors, it is time to discuss thermocouples.

Like thermistors, thermocouples are also sensitive to temperature changes, but their use, construction, and applications are slightly different.

A thermocouple, in simple words, is just a temperature detector or sensor. It is primarily used to measure the temperature at only a specific point, also called the junction. The temperature is measured as a fluctuation of current at that point.

Basic Construction

The basic construction of a thermocouple is somewhat different from a thermistor. The main construction involves connecting two different metal wires at a point.

This point is also called the junction. It is the point from which measurements are taken. You can think of it as a measuring point if you will.

Now, this junction or point can be ungrounded, grounded, or exposed. Without going into much jargon, we will explain what these terms mean.

Ungrounded means that the metal conductors or wires are not in a protective casing. They do not have any form of protection.

In grounded junctions, the wires are bonded to the protective cover and are also connected with one another.

The final type, called the exposed junction, is more suited for measuring changes in gas temperature. A cover does not protect it, and the two wires are termed as a hot junction and a cold junction.

Types of Thermocouples

When it comes to types of thermocouples, there are several different types, and each is useful in its own way.

Here, we will discuss the few main types without going into too much detail.

  • Type K

Perhaps the most common type of thermocouple out there consists of two metal wires, one made of chromium and the other made of nickel.

Both metals are excellent conductors of heat and are thus sensitive to temperature changes. This type of thermocouple is cheaper and more robust than most found in the market.

  • Type J

This one consists of metal wires made of iron. This type of thermocouple is also very popular and is widely used in labs for experiments.

It is pretty precise, as the K type, and it also has increased accuracy and reliability.

However, it does have some drawbacks. One such drawback is that it works within a much narrow temperature range. Also, it will be damaged if operated between large temperatures too frequently.

  • Type T

This type consists of copper and constantan mixtures. One of the most stable types yet, it is very suitable for use under narrow temperature ranges.

Furthermore, it is the type that is used in freezers and cryogenics applications.

  • Type E

Another thermocouple type utilizes Nickel and Constantan. It is quite stable and has been found to exhibit a higher degree of accuracy when compared to its counterparts.

It has a rather extensive and robust working range, making it suitable for large-scale industrial applications.

Where Are They Used?

Now that we covered the most basic type of thermocouples, we can take a close look at their application.

  • High-quality sensors in homes and offices, e.g., fire alarms and sensors
  • Monitoring and keeping track of temperatures in furnaces in the metalworking industry
  • Refrigerators and cryogenics
  • They are used to monitor temperature changes in chemical plants, e.g., in the petrol industry.

Thermistors vs. Thermocouples

Now it is time for the battle between these two items. While both are useful, no doubt about that; there are certain areas where one overtakes the other and certain areas where they are almost par.

This section will take a close look at these areas and highlight the main differences.

Working Temperature Range

So the first point or aspect that we want to address is the temperature range, for obvious reasons.

Since both items are basically high-quality sensors, the working temperature range defines how robust they are and can be a defining benchmark for their applications.

Thermistors usually have a working temperature range that is between -50 to 250 Celsius. It may vary slightly from brand to brand or from materials used to make it, but this range is more or less universal.

On the other hand, thermocouples have a working temperature range between -200 and 1250 degrees Celcius.

As you can see here, the thermocouple has a much broader and, therefore, more robust temperature range.

The reason for this lies in the fact that the metals used in the thermocouple are better conductors and thus more receptive over a wide temperature range.

Time of Response

In this sector, they are both rather evenly matched/ Thermistors have a response time ranging between 0.12 and 10s, and thermocouples have a response time ranging from 0.2 to 10s.

Notice that the thermistor works over a slightly larger time period. While this may not seem like much, it certainly matters when dealing with small-time changes that sensors need to pick up.

Material

This is another major area where the two types of sensors differ quite a lot.

Thermistors are made with lighter metals, such as Manganese and certain oxides, mixed to form a slurry. They also employ semiconductor materials like Gallium.

Thermocouples use metals heavier in nature, such as alloys of chromium, nickel, iron, and copper.

These may be used separately but are also used as mixtures.  Moreover, you will find that thermocouples also use rarer metals, such as rhodium and rhenium, which are not typically used in thermistors.

Main Uses

When it comes to main uses, they are both used in certain and specific areas. Thermistors are used primarily for large-scale industrial applications. It includes the cooling industry, chemical industry, and semiconductor industry.

Thermocouples are used in industries as well as homes. Applications such as ovens, fire alarms, sensors, and cryogenics all employ the use of thermocouples.

In a nutshell, thermocouples can measure the changes in temperature directly. However, thermistors do not measure the temperature directly. Their resistance change corresponds to a change in temperature.

Overall Cost

When it comes to the overall cost, the thermistor is much cheaper than the thermocouple. The main reason for this is that thermistors are not attached to an external power source and are just part of the circuitry.

However, thermocouples are attached to an external circuit, and they also require a power source to provide the driving current.

This difference in construction and working principle makes thermocouples a little more expensive.

You can quickly learn about “Thermistors Vs. RTD – What Are the Differences?” here…

Final Words

So now that we have reached the end of the thermistors vs. thermocouples battle, we hope you were thoroughly entertained and enlightened.

While both items have a large amount of physics behind their working principles, we tried to keep the jargon to a minimum to keep things practical.

Hopefully, you now know the key features of both and can also make a purchase if needed, all by yourself.

Resources:

1. www.thermocoupleinfo.com/

2. enercorp.com/types-of-thermistors/

3. https://www.omega.com/en-us/resources/thermocouples-applications

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